Dual-Layer Beam Forming in Cellular Networks

ABSTRACT

Single user and multiuser MIMO transmission in a cellular network may be performed by selecting by a base station (eNB) to transmit either one or two transmission layers. When one transmission layer is selected, a first transmission layer is precoded with a first precoder. A first demodulation reference signal (DMRS) sequence or a second DMRS sequence is selected by the eNB and precoded using the first precoder. The first transmission layer is transmitted with the selected precoded DMRS from the eNB to a user equipment (UE), and an indicator is transmitted to the UE to indicate which DMRS sequence is selected and transmitted.

CLAIM TO PRIORITY UNDER 35 U.S.C. 119

The present application claims priority to and incorporates by referenceU.S. provisional application No. 61/181,375 filed on May 27, 2009,entitled “Dual-Layer BF for SU-MIMO and DU-MIMO in Rel-9.”

FIELD OF THE INVENTION

This invention generally relates to wireless cellular communication, andin particular to multi-input multi-output transmission in orthogonal andsingle carrier frequency division multiple access (OFDMA) (SC-FDMA)systems.

BACKGROUND OF THE INVENTION

Wireless cellular communication networks incorporate a number of mobileUEs and a number of NodeBs. A NodeB is generally a fixed station, andmay also be called a base transceiver system (BTS), an access point(AP), a base station (BS), or some other equivalent terminology. Asimprovements of networks are made, the NodeB functionality evolves, so aNodeB is sometimes also referred to as an evolved NodeB (eNB). Ingeneral, NodeB hardware, when deployed, is fixed and stationary, whilethe UE hardware may be portable.

User equipment (UE), also commonly referred to as a terminal or a mobilestation, may be a fixed or mobile device and may be a wireless device, acellular phone, a personal digital assistant (PDA), a wireless modemcard, and so on. Uplink communication (UL) refers to a communicationfrom the UE to the NodeB, whereas downlink (DL) refers to communicationfrom the NodeB to the UE. Each NodeB contains radio frequencytransmitter(s) and the receiver(s) used to communicate directly with theUE, which may move freely around it. Similarly, each UE contains radiofrequency transmitter(s) and the receiver(s) used to communicatedirectly with the NodeB. In cellular networks, the UE cannot communicatedirectly with each other but have to communicate with the NodeB.

Long Term Evolution (LTE) wireless networks, also known as EvolvedUniversal Terrestrial Radio Access (E-UTRA), are being standardized bythe 3GPP working groups (WG). OFDMA (orthogonal frequency divisionmultiple access) and SC-FDMA (single carrier FDMA) access schemes werechosen for the down-link (DL) and up-link (UL) of E-UTRA, respectively.User equipment are time and frequency multiplexed on a physical uplinkshared channel (PUSCH), and a fine time and frequency synchronizationbetween UE's guarantees optimal intra-cell orthogonality. In case the UEis not UL synchronized, it uses a non-synchronized Physical RandomAccess Channel (PRACH), and the Base Station provides back someallocated UL resource and timing advance information to allow the UE totransmit on the PUSCH. The general operations of the physical channelsare described in the EUTRA specifications, for example: “3^(rd)Generation Partnership Project; Technical Specification Group RadioAccess Network; Evolved Universal Terrestrial Radio Access (E-UTRA);Physical Channels and Modulation (TS 36.211 Release 8, or later).”

Several types of physical channels are defined for the LTE downlink. Onecommon characteristic of physical channels is that they all conveyinformation from higher layers in the LTE stack. This is in contrast tophysical signals, which convey information that is used exclusivelywithin the physical (PHY) layer. Currently, the LTE DL physical channelsare as follows: Physical Downlink Shared Channel (PDSCH), PhysicalBroadcast Channel (PBCH), Physical Multicast Channel (PMCH), PhysicalControl Format Indicator Channel (PCFICH), Physical Downlink ControlChannel (PDCCH), and Physical Hybrid ARQ Indicator Channel (PHICH).

A reference signal (RS) is a pre-defined signal, pre-known to bothtransmitter and receiver. The RS can generally be thought of asdeterministic from the perspective of both transmitter and receiver. TheRS is typically transmitted in order for the receiver to estimate thesignal propagation medium. This process is also known as “channelestimation.” Thus, an RS can be transmitted to facilitate channelestimation. Upon deriving channel estimates, these estimates are usedfor demodulation of transmitted information. In downlink transmission,two types of reference signals are available. The first type ofreference signal is un-precoded and is transmitted over the entiresystem bandwidth of a cell, and is generally referred to ascell-specific reference signal (CRS). Another type of reference signalis modulated by the same precoder as applied on the data channel, andtherefore enables a UE to estimate the effective precoded MIMO channelcharacteristics, This type of RS is sometimes referred to asDe-Modulation RS or DMRS. DMRS is transmitted only when a UE is beingscheduled, and is therefore only transmitted over the frequency resourceassignment of data transmission. Note that DMRS can also be applied inuplink transmission (PUSCH), in case UE transmitter is equipped withmultiple antennas. Note that RS can also be transmitted for otherpurposes, such as channel sounding (SRS), synchronization, or any otherpurpose. Also note that Reference Signal (RS) can be sometimes calledthe pilot signal, or the training signal, or any other equivalent term.

The LTE PHY can optionally exploit multiple transceivers and antenna atboth the base station and UE in order to enhance link robustness andincrease data rates for the LTE downlink. Spatial diversity can be usedto provide diversity against fading. In particular, maximal ratiocombining (MRC) is used to enhance link reliability in challengingpropagating conditions when signal strength is low and multipathconditions are challenging. Transmit diversity can be used to improvesignal quality by transmitting the same data from multiple antennas tothe receiver. Spatial multiplexing can be used to increase systemcapacity by carrying multiple data streams simultaneously from multipleantennas on the same frequency. Spatial multiplexing may be performedwith one of the following cyclic delay diversity (CDD) precodingmethods: zero-delay, small-delay, or large-delay CDD. Spatialmultiplexing may also be referred to as MIMO (multiple input multipleoutput).

With MRC, a signal is received via two (or more) separateantenna/transceiver pairs. The antennas are physically separated, andtherefore have distinct channel impulse responses. Channel compensationis applied to each received signal within the baseband processor beforebeing linearly combined to create a single composite received signal.When combined in this manner, the received signals add coherently withinthe baseband processor. However, the thermal noise from each transceiveris uncorrelated, resulting in improved signal to noise ratio (SNR). MRCenhances link reliability, but it does not increase the nominal systemdata rate since data is transmitted by a single antenna and is processedat the receiver via two or more receivers. MRC is therefore a form ofreceiver diversity rather than more conventional antenna diversity.

MIMO, on the other hand, does increase system data rates. This isachieved by using multiple antennas on both the transmitting andreceiving ends. In order to successfully receive a MIMO transmission,the receiver must determine the channel impulse response from eachtransmitting antenna. In LTE, channel impulse responses are determinedby sequentially transmitting known reference signals from eachtransmitting antenna. While one transmitter antenna is sending thereference signal, the other antenna is idle. Once the channel impulseresponses are known, data can be transmitted from both antennassimultaneously. The linear combination of the two data streams at thetwo receiver antennas results in a set of two equations and twounknowns, which is resolvable into the two original data streams.

Physical channels are mapped to specific transport channels. Transportchannels are service access points (SAPs) for higher layers. Eachphysical channel has defined algorithms for bit scrambling, modulation,layer mapping, precoding, and resource assignment. Layer mapping andprecoding are related to MIMO applications. Basically, a layercorresponds to a spatial multiplexing channel. Channel rank can varyfrom one up to the minimum of number of transmit and receive antennas.For example, given a 4×2 system, i.e., a system having four transmitantennas and two receive antennas, the maximum channel rank is two. Thechannel rank associated with a particular connection varies in time andfrequency as the fast fading alters the channel coefficients. Moreover,the channel rank determines how many layers, also referred to as thetransmission rank, can be successfully transmitted simultaneously. Forexample, if the channel rank is one at the instant of the transmissionof two layers, there is a strong likelihood that the two signalscorresponding to the two layers will interfere so much that both of thelayers are erroneously detected at the receiver. In conjunction withprecoding, adapting the transmission to the channel rank involvesstriving to use as many layers as the channel rank. Layer mappingspecifies exactly how the extra transmitter antennas are employed. Theprecoding applied for the demodulation reference signal (DMRS) is thesame as the one applied for the PUSCH (for uplink) and PDSCH (fordownlink). Cyclic shift separation is the primary multiplexing scheme ofthe demodulation reference signals.

Precoding is used in conjunction with spatial multiplexing. MIMOexploits multipath to resolve independent spatial data streams. In otherwords, MIMO systems require a certain degree of multipath for reliableoperation. In a noise-limited environment with low multipath distortion,MIMO systems can actually become impaired. The basic principle involvedin precoding is to mix and distribute the modulation symbols over theantennas while potentially also taking the current channel conditionsinto account. Precoding can be implemented by, for example, multiplyingthe information carrying symbol vector containing modulation symbols bya matrix which is selected to match the channel based on a certainselection criterion. Some examples of selection criterion includeaverage throughput and maximum signal-to-interference-noise ratio(SINR). Sequences of symbol vectors thus form a set of parallel symbolstreams and each such symbol stream is referred to as a “layer”. Thus,depending on the choice of precoder in a particular implementation, alayer may directly correspond to a certain physical antenna or a layermay, via the precoder mapping, be distributed onto several physicalantennas.

In LTE Rel-8, single layer beamforming on antenna port 5 is alreadysupported. Single-layer beamforming is based on non-codebook precodingand relies on a dedicated demodulation reference symbol (DMRS) for datademodulation. DMRS symbols are precoded with the same precoding matricesas the PDSCH data symbols and therefore enable UE to estimate the“effective” channel after precoding. Rank-1 transmission is enforced. AUE is restricted to receive a single transport block (codeword) which ismapped to one layer (data stream) in DL transmission. From the UE'sperspective, the effective 1-layer channel appears as if data istransmitted from a single virtual antenna. DMRS corresponding to thislayer is defined as antenna port 5 in LTE Rel-8 to enable channelestimation.

BRIEF DESCRIPTION OF THE DRAWINGS

Particular embodiments in accordance with the invention will now bedescribed, by way of example only, and with reference to theaccompanying drawings:

FIG. 1 is a pictorial of an illustrative telecommunications network inwhich an embodiment of the invention is used to support single user andmultiuser MIMO transmission signals;

FIG. 2 is an illustrative format of one subcarrier (tone) of a DLtransmission subframe for use in the network of FIG. 1;

FIGS. 3 and 4 illustrate a resource block with DMRS pattern for atransmission layer on antenna port 7 and port 8;

FIG. 5 is a flow diagram illustrating selection and transmission of oneor two transmission layers;

FIG. 6 is a flow diagram illustrating reception of one or twotransmission layers;

FIG. 7 is a block diagram of an illustrative transmitter fortransmission of a MIMO signal in the network of FIG. 1 according to anembodiment of the invention; and

FIG. 8 is a block diagram illustrating an exemplary portion of acellular network with a base station in communication with a mobiledevice.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

Embodiments of the invention support single user (SU) dual-layerbeamforming using UE specific RS for both LTE-TDD (time division duplex)and FDD (frequency division duplex) using UE specific demodulationreference signals and mapping of physical data channel to resourceelements that may provide forward compatibility with LTE-A DemodulationRS (DMRS). Embodiments of the invention extend single user dual-layerbeamforming to multi-user dual-layer beamforming, as will be describedin more detail below.

FIG. 1 is a pictorial of an illustrative telecommunications network 100in which an embodiment of the invention is used to support single userand multiuser MIMO transmission signals, as described in more detailbelow. The illustrative telecommunications network includes eNBs 101,102, and 103, though in operation, a telecommunications network mayinclude many more eNBs or fewer eNBs. Each of eNB 101, 102, and 103 isoperable over corresponding coverage areas 104, 105, and 106. Each eNB'scoverage area is further divided into cells. In the illustrated network,each eNB's coverage area is divided into three cells. Handset or otherUE 109 is shown in Cell A 108, which is within coverage area 104 of eNB101. Transmission occurring between eNB 101 and UE 109 via downlinkchannel 110 and uplink channel 112. As UE 109 moves 116 out of Cell A108, and into Cell B 107, UE 109 may be “handed over” to eNB 102.

When UE 109 is not up-link synchronized with eNB 101, non-synchronizedUE 109 employs non-synchronous random access (NSRA) to requestallocation of up-link 112 time or frequency or code resources. If UE 109has data ready for transmission, for example, traffic data, measurementsreport, tracking area update, etc., UE 109 can transmit a random accesssignal on up-link 112 to eNB 101. The random access signal notifies eNB101 that UE 109 requires up-link resources to transmit the UE's data.ENB 101 responds by transmitting to UE 109, via down-link 110, a messagecontaining the parameters of the resources allocated for UE 109 up-linktransmission along with a possible timing error correction. Afterreceiving the resource allocation and a possible timing adjustmentmessage transmitted on down-link 110 by eNB 101, UE 109 may adjust itstransmit timing, to bring the UE 109 into synchronization with eNB 101,and transmit the data on up-link 112 employing the allotted resourcesduring the prescribed time interval. eNB 101 also sends a downlink grantto UE 109 when the eNB has data to transmit to UE 109. The downlinkgrant specifies one or more resource blocks on which the eNB willtransmit to the UE on downlink 110.

Similarly, UE 117 may communicate with eNB 101 on downlink 111 anduplink 113. eNB 101 may decide send data on DL 110 in SU-MIMO mode to UE109. Alternatively, eNB 101 may decide to send data on DL 110 to UE 109and on DL 111 to UE 117 in MU-MIMO mode, as will be described in moredetail below.

FIG. 2 is an illustrative format of one subcarrier (tone) of a DLtransmission subframe for use in the network of FIG. 1. It comprises of14 resource elements. Elements of the present invention will bedescribed in the context of EUTRA sub-frame, even though itsapplicability is broader. Orthogonal frequency division multiple access(OFDMA) based systems include classic OFDMA as well as its alternatives,like single carrier frequency division multiple access (SC-FDMA) anddiscrete Fourier transform (DFT)-spread OFDMA. In OFDMA based systems,frequency resources are divided into tones. Tones are further groupedinto “tone blocks” or “resource blocks” for purposes offrequency-dependent scheduling of mobiles, and other possible purposes.Thus, each mobile can be allocated one or more resource blocks in anOFDMA based system. This group of resource blocks will be denoted as thefrequency allocation for a given mobile.

FIG. 2 illustrates just one subcarrier of sub-frame 200 comprising twoslots 201 and 202. It comprises of 14 resource elements. This resourceelement is configured for use on antenna port 7 or 8, as will bedescribed in more detail below. Duration of the EUTRA sub-frame is 1 ms,which means that duration of two slots 201 and 202 is 0.5 ms each. Eachslot comprises seven symbols when a normal cyclic protection field (CP)is appended to each symbol, or six symbols when an extend CP is appendedto each symbol. For example, slot 201 comprises symbols 203-209. Theslot 202 comprises symbols 210-216. Symbols 208, 209, 215 and 216 areDemodulation (DM) Reference Signals (RS), and are used to derive channelestimates which are needed for coherent demodulation of the remainingsymbols that are modulated with payload data. LTE Rel 9 also definesseveral other antenna port configurations for antenna ports 0-3 and 5,where port 0-3 are unprecoded CRS antenna ports and port 5 is DMRS forsingle-layer data transmission defined in Rel-8. Each symbol has a timeduration equal to approximately T, which is a function of the slot time.In this embodiment, the slot time is 500 μsec. Since the first symbol inthe slot has more cyclic prefix samples, not all symbols are exactlyequal in duration, as per 3GPP TS36.211. Nevertheless, all symbols canbe considered to be approximately equal in duration, which doesn'texceed 75 μsec. Note that if all symbols were exactly equal in duration,the symbol time T would approximately be equal to 500 μsec/7=71.4 μsec.

In some embodiments of the invention, the set of reference signalsequences comprises CAZAC sequences and near—CAZAC sequences. Near—CAZACis a term which designates sequences which are obtained using computersearch methods, and whose properties approximate CAZAC properties. Insome embodiments of the invention, CAZAC sequences are Zadoff—Chusequences. In some embodiments of the invention, near—CAZAC sequencesare sequences of the form exp(j*π*φ(n)/4); wherein the length of φ(n) isan integral multiple of 12. Here, “j” is the imaginary unit.

In some embodiments of the invention, the set of reference signalsequences comprises CAZAC sequences only. In some embodiments of theinvention, the set of reference signal sequences comprises near—CAZACsequences only. In some embodiments of the invention, the set ofreference signal sequences comprises both CAZAC sequences and near—CAZACsequences. Sometimes, a phase ramp is applied to modify the firstsequence, for example exp(j*n*α+j*π*φ(n)/4) can still be considered as areference signal sequence. For 3GPP EUTRA, there are 30 possiblesequences of length 24, which are also near—CAZAC. For length 36 andmore, sequences are produced from CAZAC sequences. Thus, the set ofreference signal sequences comprises both CAZAC and near—CAZACsequences.

Further details on the construction of reference signals, demodulationreference signals and sounding reference signals are included in 3rdGeneration Partnership Project; GPP TS 36.211 V9.1.0 (2010) “TechnicalSpecification Group Radio Access Network; Evolved Universal TerrestrialRadio Access (E-UTRA); Physical Channels and Modulation,” in particularin section 6 and which is incorporated herein by reference.

FIGS. 3 and 4 illustrate a resource block with DMRS pattern for atransmission layer on antenna port 7 and port 8, respectively. Eachresource block includes a set of resource elements, where one resourceelement is a resource grid uniquely defined by one subcarrier and oneOFDM symbol. For normal CP, one resource block has 12 subcarriers and 14OFDM symbols, hence it comprises of 12×14=168 resource elements. A groupof resource element 310 is representative and was described in moredetail with reference to FIG. 2. FIG. 3 illustrates a resource block(RB) with a normal CP for downlink transmission on antenna port 7. EachRB has twelve DMRS (R₇), distributed in four OFDM symbols where eachOFDM symbol has three DMRS. FIG. 4 illustrates an RB with normal CP fordownlink transmission on antenna port 8. Each RB has twelve DMRS (R₈),distributed in four OFDM symbols where each OFDM symbol has three DMRS.The twelve DMRS symbols within the RB supporting beamforming on antennaports 7 and 8 are demodulation reference symbols for PDSCH transmissionin the RB.

Two types of MIMO precoding are standardized in LTE Rel-8.

The first type is codebook-based precoding with CRS, where precodingmatrixes are confined within a given codebook. Spatial multiplexing issupported with multiple layers in PDSCH. PDSCH demodulation is based onCRS.

The second type is non-codebook-based precoding with DMRS whereprecoding matrices are not necessarily within a codebook and can bearbitrary. 1-layer beamforming with DMRS is supported. PDSCHdemodulation is based on DMRS.

While LTE Rel-8 supports single layer beamforming with DMRS, no explicitsupport of multi-user (MU) MIMO with single-layer beamforming isprovided. However multi-user beamforming is explicitly supported in LTERel-8 as two or more UEs in transmission mode 7 can be scheduled to thesame physical resource block (PRB). This is a pure eNB implementationissue and is transparent to the UE. In case grid of beam (GoB) type ofbeamforming is applied, there will be good attenuation between differentusers allocated to different beams so that the performance of DMRS basedchannel estimation at the UE is not impacted. The effective DMRS densityper user is not reduced which would have been the case if LTE-A type oforthogonal DMRS is used.

For example: four orthogonal beams may be created to cover one sector.Each beam can support one UE with single-layer beamforming, hence fourUEs can be implicitly supported in LTE-Rel-8 MU-MIMO mode. With DMRSsingle-layer beamforming, MU-MIMO in LTE Rel-8 is an eNB implementationissue and transparent to UE, no extra signaling about the presence ofother UE is provided in the L3 radio resource control (RRC) signaling orL1/L2 control signaling. Reporting of channel quality indicator (CQI)and precoding matrix index (PMI) is supported. Codebook based precodingis the baseline of PMI report where Rel-8 2TX codebook is assumed forPMI reporting.

Rel-8 MU-MIMO is based on codebook based precoding where the precodingmatrix for each user/layer is selected from a predefined codebook. Asingle codeword is transmitted to each user where the codeword is mappedto a single layer. Because the limited flexibility of UE pairing andless-refined CQI report results in less optimal link adaptation, theperformance gain of MU-MIMO in Rel-8 over SU-MIMO is expected to belimited.

Compared to codebook-based precoding where precoding matrices areselected from a finite set of pre-defined matrices (codebook), precodingmatrices used in non-codebook precoding can be chosen more flexibly andtherefore are more flexible to achieve a precoding gain. To enable a UEto estimate the precoding matrices and the downlink channel, UE-specificDMRS is used for data demodulation where the DMRS symbols are precodedby the same precoding matrices applied on the data. Hence the UE is ableto estimate the “effective” downlink channel—a combination of actualdownlink physical channel and precoding matrices—to perform datadecoding.

DMRS supporting non-codebook based precoding is already specified in LTERel-8 for single-layer beamforming, where only a single spatial layer(e.g. data stream) is transmitted to a UE. In addition, DMRS will onlybe present in the physical resources of downlink data transmission thatis scheduled for a UE. In addition, the frequency-domain position ofDMRS for a UE will be offset by a cell-specific parameter, e.g. Cell-IDin Rel-8. This is to ensure randomization of the frequency domainposition of the DMRS symbols in different cells as to reduce inter-cellinterference. Similarly, cell-specific reference signal (CRS) in Rel-8is also offset by the cell-ID in Rel-8, as to randomize the CRS positionand reduce inter-cell interference.

There are currently two definitions of “transparency”. A first type oftransparency is defined in terms of SU/MU-MIMO mode. Non-transparencymeans that a UE is semi-statically configured in either the SU orMU-MIMO mode which is signaled to the UE via higher layer signaling. UEis semi-statically configured to operate in SU-MIMO transmission mode orMU-MIMO transmission mode, hence different DL (downlink) control and UEfeedback are used. Higher layer (L3) radio resource control (RRC)signaling configures the SU/MU-MIMO mode for the UE. Transparency meansthat the UE is configured in one joint SU/MU MIMO transmission mode,thus the same UE feedback and DL control.

A second type of transparency is defined in terms of knowledge of theco-scheduled UE. Non-transparency means that the presence of aco-scheduled UE is known and possibly taken into account in the DLcontrol signaling and UE feedback. On the other hand, non-transparencymeans that UE is agnostic about the presence of a co-scheduled UE, hencethe same UE feedback and DL control is used.

Regardless of which definition is used, “transparency” essentially meansthat no signaling is provided to the UE regarding transmission to theother UEs in the same time/frequency resources. If the presence of aco-scheduled UE is provided to the target UE, MU-MIMO transmission is“non-transparent”. For example, in LTE Rel-8, MU-MIMO is asemi-statically configured transmission mode, hence a UE will know thatit will be paired up with another UE if it is configured in such atransmission mode. Another possibility is to signal to the UEdynamically whether or not transmission to another UE is present, thatis, a dynamic downlink grant has been provided on the PDCCH. In thiscase, a UE can be either configured in the semi-static SU or MU-mode.

Dynamic rank-adaptation is already supported for SU dual-layerbeamforming in LTE Rel-9, therefore a UE can receive rank-1 or rank-2beamforming. This can be done with a 1-bit transmit rank indicator (TRI)value included in the DL grant, or by signaling of theenabling/disabling of one of the two transport blocks in the DL grant.For MU-MIMO in LTE Rev-8, a UE is restricted to receive rank-1 and hencethe TRI field is not required in the DL grant.

If SU/MU mode is RRC signaled, the UE need to be semi-staticallyconfigured in a SU or MU mode. As a result, the MU-MIMO mode needs todefine a different DL DCI (downlink control information) format thanSU-MIMO dual-layer beamforming.

In an embodiment of the invention, the DL grant for MU-MIMO may need toinclude a 1-bit indicator of the index of the layer, which is needed foridentifying the associated DMRS for channel estimation and rank-1 PDSCHdecoding.

Semi-Static SU-MIMO and MU-MIMO Mode for Dual-Layer Beamforming

In this approach, a UE is semi-statically configured by higher layer RRCsignaling in either the SU or MU-MIMO mode. For SU and MU-MIMO mode,different downlink grant (DCI) formats and different UE CQI (channelquality information) report schemes are used.

An exemplary DCI format in response to SU-MIMO dual-layer beamformingmay include the precoding-related control fields listed in Table 1.

TABLE 1 DCI format for SU-MIMO dual layer beamforming in a semi-staticenvironment Control fields Number of bits Transmit rank indicator (RI) 1TBS1 - MCS (modulation and coding 5 scheme) TBS1 - NDI (new dataindicator) 1 TBS1 - RV (redundancy version) 2 TBS2 - MCS 5 TBS2 - NDI 1TBS2 - RV 2 Codeword-to-layer swapping flag 1

As shown in Table 1, a 1-bit transmit rank indicator (TRI) is used tosignal the number of layers in DL transmission. For example, TRI=1 meansrank-1 (1 layer, 1 codeword) is transmitted to a UE, while TRI=2 meansrank-2 (2 layers, 2 codewords) are transmitted to a UE. All DL layersare transmitted to the same user. A five bit MCS (modulation and codingscheme) field, a one bit NDI (new data indicator) and a two bit RV(redundancy version) is provided for transport block 1 and for transportblock 2.

A one bit transport-block to codeword swapping flag is also provided.This bit indicates the mapping between transport blocks and codewords,which can be either: TB 1−>codeword 1, TB−2>codeword 2; or TB1−>codeword 2, TB−2>codeword 1.

For TRI=1 single-layer (rank-1) transmission, either the first or thesecond transport block is disabled (i.e. not transmitted). This can besignaled by MCS=0 and RV=1 for the corresponding transport block, asused in Rel-8.

In this scheme, precoding fields are not required, since precoding isnon-codebook based.

Alternatively, for SU-MIMO dual-layer beamforming, the DL grant mayre-use DCI format 2 or 2A in LTE Rel-8 codebook-based spatialmultiplexing. In this case, the precoding fields in DCI 2/2A in Rel-8are reserved. Alternatively, a new DCI format may be designed fordual-layer beamforming SU-MIMO by removing unused control fields of DCI2/2A.

For semi-statically configured MU-MIMO mode dual-layer beamforming, amaximum of two codewords/layers may be transmitted simultaneously on thesame frequency resources to two different UEs. The DCI format inresponse to MU-MIMO in includes the control fields listed in Table 2.

TABLE 2 DCI format for MU-MIMO dual layer beamforming in a semi-staticenvironment Control fields Number of bits MCS 5 NDI 1 RV 2 Power sharingindication 1 DMRS layer indication 1

Since each UE is restricted to receive rank-1 single-layer, no explicitsignaling of the TPI is included in the DCI format. A five bit MCS(modulation and coding scheme) field, a one bit NDI (new data indicator)and a two bit RV (redundancy version) is provided for one transportblock.

Precoding fields are not required, since precoding is non-codebookbased. The codeword for each layer is implicit for that layer.

Since total DL transmit power is shared by two UEs, it is possible toinclude 1-bit power sharing information in the DCI format for MU-MIMO,as follows:

Bit=0: DL EPRE (energy per resource element) of PDSCH equals to rho,where rho is the semi-statically configured DL PDSCH EPRE (dB) for theUE.

Bit=1: DL EPRE of PDSCH equals to rho-3, where rho is thesemi-statically configured DL PDSCH EPRE (dB) for the UE.

This power sharing bit can also be understood as an implicit signalingof the presence of the co-scheduled UE. When a co-scheduled UE ispresent, the transmit power to each UE is reduced by half (3 dB).

Additionally, the UE also needs to know which one of the two layerscarries the target PDSCH data to itself in order to properly performdownlink channel estimation and demodulation. Therefore, a 1-bitsignaling of the index of the DMRS layer is required to indicate whichof the two layers is targeting this UE, and on which layer the UE shoulddemodulate the DMRS.

Dynamic SU/MU-MIMO Switching for Dual-Layer Beamforming

In the case of dynamic SU/MU-MIMO switching, a UE receives its SU/MIMOconfiguration message in the DL grant and it is possible for the UE tofast switch between SU/MU transmissions via L1/L2 control signaling(e.g. PDCCH). A common DL grant is used for both SU/MIMO transmissions.The control fields in the DCI format may include information similar tothat listed in Table 1.

As listed in Table 1, a one bit transmit rank indicator (TRI) to signalthe number of layers in DL transmission is included in the DCI. TRI=1indicates 1 layer (1 codeword) is transmitted in the DL, and TRI=2indicates two layers (two codewords) are transmitted in the DL. Notethat these layers can be transmitted to one UE or two UEs respectively.A five bit MCS (modulation and coding scheme) field, a one bit NDI (newdata indicator) and a two bit RV (redundancy version) is provided fortransport block 1 and for transport block 2.

A one bit transport-block to codeword swapping flag is optionallyprovided. This bit indicates the mapping between transport blocks andcodewords. If 1-bit transport block to codeword swapping flag is notincluded, a fixed mapping between transport block and codeword isassumed.

In this scheme, precoding fields are not required, since precoding isnon-codebook based. Note that a fixed mapping rule can be applied on thecodeword to DMRS mapping so that the codeword is implicit for eachlayer. For example, codeword 1 is always transmitted and demodulatedwith DMRS sequence 1 (port 7), codeword 2 is always transmitted anddemodulation with DMRS sequence 2 (port 8). This applies regardless ofwhether TDM/FDM multiplexing or CDM multiplexing is configured for thetwo DMRS layers.

In order for one DL grant format to work for both SU/MU-MIMO modes, theDL grant possesses information about which mode (SU or MU) the UE isexpected to operate in. This could be implemented in a few ways.

In a first embodiment, a one bit flag of SU/MU switching is explicitlyincluded in each DL grant. This bit indicates that the correspondingPDSCH are for a single user (SU) or for two users (MU).

The transport block to codeword mapping is signaled by one bit in the DLgrant, and an explicit fixed mapping between codeword to DMRS layer isassumed, i.e., codeword 1 is transmitted on DMRS layer 1 and codeword 2is transmitted on DMRS layer 2. If SU-MIMO is detected (1-bit in PDCCH),the relationships between the transport blocks and the DMRS layers areclearly defined.

In the case of MU-MIMO, rank-1 single-layer is received by each UE.Signaling the index to the DMRS layer may not be needed. The index tothe DMRS layer may be implicitly derived in the following manner aslisted in Table 3.

TABLE 3 Derivation of index to DMRS layer from rank indicator (RI) forMU-MIMO dual layer beamforming with dynamic switching SU/MU RI Two TBSsInterpretation SU RI = 1 Enabled, Clear disabled SU RI = 2 Enabled,Clear enabled MU RI = 1 Enabled, UE may need to assume that the 1^(st)TBS (transport block enabled size) is always the desired TBS and the2^(nd) TBS is associated with the co-scheduled UE. Index of the desiredDMRS layer is easily obtained from TB to codeword swapping flag. The TBSfields of the second TBS can be used to signal the MCS of theinterference signal, useful for the target UE's interference estimationor SIC cancellation. MU RI = 1 Enabled, UE assumes the enabled TBS isfor itself. Index of the disabled desired DMRS layer is easily obtainedfrom the enabled TBS and TB-to-codeword swapping flag. The 1-bit NDI ofthe disabled TBS may be used to signal the presence of transmission tothe co-scheduled UE. For example, NDI = 0 indicates that there is noco-scheduled UE in the same subframe, while NDI = 1 indicates that thereis a co-scheduled UE in the same subframe. Such information is helpfulfor the UE to correctly interpret the PDSCH EPRE, to performinterference estimation and nullification to improve MU-MIMOperformance. MU RI = 2 Enabled, UE may need to assume that the 1^(st)TBS is always the enabled desired TBS and the 2^(nd) TBS is for theco-scheduled UE. Index of the desired DMRS layer is obtained from TB tocodeword swapping flag. The TBS fields of the second TBS can be used tosignal the MCS of the interference signal, useful for the target UE'sinterference estimation or SIC cancellation. MU RI = 2 Enabled, UEassume the enabled TBS is for itself. Index of the desired disabled DMRSlayer is easily obtained from the enabled TBS and TB- to-codewordswapping flag. 1-bit NDI of the disabled TBS may be used to signal thepresence of transmission to the co- scheduled UE. For example, NDI = 0indicates that there is no co-scheduled UE in the same subframe, whileNDI = 1 indicates that there is a co-scheduled UE in the same subframe.

Alternatively, the DMRS layer of the target rank-1 PDSCH may beexplicitly signaled (using 1-bit, for example) in the DL grant. TBS(transport block size) fields of the interfering transport block (e.g.MCS level) could be used for interference estimation and cancelation.For example, for an advanced receiver which is capable to performsuccessive interference cancellation, it may use the MCS level of theinterference signal to decode the other stream (interference) first,subtract it from the received signal to improve the decoding reliabilityof its own data stream.

In a second embodiment, SU/MU switching may be explicitly signaled byother approaches, e.g., masking the CRC of the PDCCH with differentscrambling sequences. For example, scrambling sequence 1=UE shallreceive both transport blocks (SU-MIMO). Scrambling sequence=0 means UEshould receive only one transport block (MU-MIMO).

In this case, the interpretation of DMRS layer, presence of theco-scheduled UE, MCS of the co-scheduled UE can be similarly obtained asin Table 3. Index of the DMRS layer may be either derived with RI+TBSenabling/disabling configuration as in Table 3, or explicitly signaledin DL grant.

In a third embodiment, it is also possible to apply more scramblingsequences to the PDCCH CRC to further differentiate the index of theDMRS layer that a UE will use to demodulate PDSCH when configured inSU-MIMO transmission. However, this reduces the UE-ID space andnegatively impacts the system capacity.

In other embodiments of the invention, for dynamic SU/MU mode with thesame PDCCH format, some information may also be implicitly signaled bydifferent masking of the CRC of the PDCCH with different scramblingsequences. This includes: presence of a co-scheduled UE, indexing to theDMRS layer, SU/MU-MIMO mode, or combinations of the above.

An embodiment of the invention is specified in 3rd GenerationPartnership Project; Technical Specification Group Radio Access Network;Evolved Universal Terrestrial Radio Access (E-UTRA); Physical Channelsand Modulation (Release 9) 3GPP TS 36.211 V9.1.0 (2010-03)), section6.3.3.4. Spatial multiplexing using antenna ports with UE-specificreference signals supports two antenna ports and the set of antennaports used is pε{7,8}. For transmission on two antenna ports, pε{7,8},the precoding operation is defined by:

$\begin{bmatrix}{y^{(7)}(i)} \\{y^{(8)}(i)}\end{bmatrix} = \begin{bmatrix}{x^{(0)}(i)} \\{x^{(1)}(i)}\end{bmatrix}$where  i = 0, 1, …  , M_(symb)^(ap) − 1,  M_(symb)^(ap) = M_(symb)^(layer).

3rd Generation Partnership Project; Technical Specification Group RadioAccess Network; Evolved Universal Terrestrial Radio Access (E-UTRA);Multiplexing and channel coding (Release 9) 3GPP TS 36.212 V9.1.0(2010-03) contains a description of DCI format 2B in section 5.3.3.1.5Bin accordance with aspects described herein.

FIG. 5 is a flow diagram illustrating operation of MIMO transmissionwith eNB selection of transmission layers. The eNB selects 502 thenumber of transmission layers based on available resources and amount ofdata that needs to be transmitted. The eNB also decides whether totransmit one UE in SU-MIMO or to transmit to two, or more, UE inMU-MIMO. When one layer is selected 504, the eNB precodes 510 a firsttransmission layer with a first precoder. Since non-codebook precodingis being used in this mode, the eNB may select a precoder based onchannel conditions or other considerations.

The eNB produces a first DMRS using a first cyclic shift and a basesequence. It may also produce a second DMRS using a second cyclic shiftand base sequence. These reference signals may be produced usingCAZAC-like sequences, as described in more detail earlier.

Once the reference sequences are produced, the eNB selects either thefirst DMRS or the second DMRS and precodes 511 it using the firstprecoder. Herein the first DMRS sequence corresponds to DMRS antennaport 7, and the second DMRS sequence corresponds to DMRS antenna port 8.The eNB then transmits 512 the first transmission layer on antenna port7 or 8 along with the precoded DMRS to a UE using resource blocks asdescribed with reference to FIGS. 2-4.

The eNB also transmits 513 an indicator to the UE to indicate which DMRSsequence is selected and transmitted. In this embodiment, the indicatoris a single bit that is included in a downlink grant that is sent to theUE to prepare it to receive the transmission layer from the eNB. Thedownlink grant also indicates that only one transmission layer is beingused via the rank indicator (RI) bit or though transport blockenabling/disabling mechanism in the DCI. The downlink grant may alsoindicate SU or MU mode.

When two transmission layers are selected 506, then the eNB precodes 520a first transmission layer with a first precoder, and precodes 520 afirst DMRS sequence using the first precoder. The eNB precodes 521 asecond transmission layer with a second precoder, and precodes 521 asecond DMRS sequence using the second precoder.

The eNB transmits 522 the first transmission layer and the first DMRSsequence to the UE, and also transmits 523 the second transmission layerand the second DMRS sequence to the UE. In this case, the associateddownlink grant DCI optionally included an RI that indicates twotransmission layers.

As described above, a fixed mapping rule is applied on the codeword toDMRS mapping so that the codeword is implicit for each layer. Forexample, codeword 1 is always transmitted and demodulated with DMRSsequence 1, codeword 2 is always transmitted and demodulation with DMRSsequence 2.

FIG. 6 is a flow diagram illustrating reception by a UE of one or twotransmission layers in MIMO transmission. The UE receives 602 from aneNB a downlink grant with DCI granting the UE one transmission layer ortwo transmission layers on PDSCH using an explicit RI indicator orthrough transport block enabling/disabling indication in DCI. When onetransmission layer is indicated 604, the UE receives 610 an indicatorfrom the eNB to indicate which demodulation reference signal (DMRS)sequence was selected by the eNB, i.e. a first DMRS sequence (port 7) ora second DMRS sequence (port 8). The UE has foreknowledge of thepossible sequences and now knows the exact sequence to expect in theDMRS, and on which antenna port (7 or 8) to perform channel estimation.In this embodiment, the indicator is a one bit field in the DCI of thedownlink grant. The UE then receives 611 a first transmission layer fromthe eNB with the selected DMRS. Since the UE knows the expected DMRSsequence, it uses this knowledge to assist in demodulating 612 the DMRSto determine the precoding matrix, which is a non-codebook precoder. TheUE then uses this information to demodulate 612 the first transmissionlayer, wherein the first transmission layer and the selected DMRS have asame precoding.

When two transmission layers are indicated 606, the UE receives 620 fromthe eNB a first transmission layer and a first DMRS having a firstsequence. It also receives 621 from the eNB a second transmission layerand a second DMRS having a second sequence. It has foreknowledge of thepossible DMRS sequences and when two transmission layers are received afixed mapping is assumed, so it knows the sequence for each of the twoDMRS. Using this information, the UE demodulates 622 the first DMRS andthe first transmission layer, wherein the first transmission layer andthe selected DMRS have the same precoding. The UE also demodulates 623the second DMRS and the second transmission layer, wherein the secondtransmission layer and the selected DMRS have the same precoding.

FIG. 7 is a block diagram of an illustrative transmitter 700 fortransmission of a MIMO signal. A baseband signal representing a downlinkphysical channel is formed by providing a stream of code words 702 a, bto scrambling logic 704 a, b. In this embodiment, there are twotransmission layers illustrated which are indicated by 702 a and 702 b,etc. Other embodiments may have additional layers.

Scrambling logic 704 a, b scrambles the coded bits in each of the codewords to be transmitted on a physical channel. The scrambled bits arethen provided to modulation mapper logic 706 a, b which maps thescrambled bits to modulation constellations to generate complex-valuedmodulation symbols. For example, the PUSCH may use one of the followingmodulation schemes: QPSK (quaternary phase shift keying), 16QAM(quaternary amplitude modulation), or 64QAM.

The modulated symbols are then provided to layer mapping logic 708 formapping of the complex-valued modulation symbols onto one of severaltransmission layers. The number of layers v is less than or equal to thenumber of antenna ports P used for transmission of the physical channel.The resulting complex-valued modulation symbols on each layer are thenprecoded for transmission on the antenna ports.

The complex-valued modulation symbols for each antenna port and the DMRSfor each antennae are then mapped to resource elements in resourceelement mappers 712 a, b. DMRS generators 716 a, b generate the DMRS, asdescribed in more detail above.

The resource mapped symbols are then provided to OFDM signal generationlogic 714 a, b for the generation of complex-valued time-domain OFDMsignals 718 a, b for each antenna port.

System Example

FIG. 8 is a block diagram illustrating an exemplary portion of thecellular network of FIG. 1. As shown in FIG. 8, the wireless networkingsystem 1000 includes a UE device 1001 in communication with an eNB 1002.The UE device 1001 may represent any of a variety of devices such as aserver, a desktop computer, a laptop computer, a cellular phone, aPersonal Digital Assistant (PDA), a smart phone or other electronicdevices. In some embodiments, the electronic UE device 1001 communicateswith the eNB 1002 based on a LTE or E-UTRA protocol. Alternatively,another communication protocol now known or later developed can be used.

As shown, the UE device 1001 includes a processor 1003 coupled to amemory 1007 and a Transceiver 1004. The memory 1007 stores (software)applications 1005 for execution by the processor 1003. The applications1005 could be any known or future application useful for individuals ororganizations. As an example, such applications 1005 could becategorized as operating systems (OS), device drivers, databases,multimedia tools, presentation tools, Internet browsers, e-mailers,Voice-Over-Internet Protocol (VOIP) tools, file browsers, firewalls,instant messaging, finance tools, games, word processors or othercategories. Regardless of the exact nature of the applications 1005, atleast some of the applications 1005 may direct eNB (base-station) 1002to transmit DL signals to UE device 1001 periodically or continuouslyvia the transceiver 1004.

Transceiver 1004 includes uplink logic which may be implemented byexecution of instructions that control the operation of the transceiver.Some of these instructions may be stored in memory 1007 and executedwhen needed. As would be understood by one of skill in the art, thecomponents of the uplink and downlink logic may involve the physical(PHY) layer and/or the Media Access Control (MAC) layer of thetransceiver 1004. Transceiver 1004 includes two or more receivers 1020and two or more transmitters 1022 for SU/MU-MIMO, as described in moredetail above.

eNB 1002 includes a Processor 1009 coupled to a memory 1013 and atransceiver 1010. Memory 1013 stores applications 1008 for execution bythe processor 1009. The applications 1008 could be any known or futureapplication useful for managing wireless communications. At least someof the applications 1008 may direct the base-station to managetransmissions to or from the user device 1001.

Transceiver 1010 includes an resource manager which enables eNB 1002 toselectively allocate uplink PUSCH resources and downlink PDSCH resourcesto the user device 1001. As would be understood by one of skill in theart, the components of the resource manager 1012 may involve thephysical (PHY) layer and/or the Media Access Control (MAC) layer of thetransceiver 1010. Transceiver 1010 includes a Receiver 1011 forreceiving transmissions from various UE within range of the eNB andtransmitter 1014 for transmission to the various UE within range. Theresource manager executes instructions that control the operation oftransceiver 1010. Some of these instructions may be located in memory1013 and executed when needed. The resource manager controls thetransmission resources allocated to each UE that is being served by eNB1002 and broadcasts control information via the physical downlinkcontrol channel PDCCH.

During MIMO transmission from eNB 1002 via transmitters 1014 on PDSCH,eNB 1002 monitors channel conditions to adapt to the prevailingcondition. This includes monitoring the channel quality indicator (CQI)feedback provided by UE 1001 on the uplink channel using conditionmonitoring logic 1012 that is coupled to receiver 1011.

During MIMO transmission to UE 1001 via transmitters 1014 on PDSCH, eNB1002 forms DMRS signals using different amounts of cyclic shift,depending on the number of layers being used for transmission, asdescribed in more detail above.

A typical eNB will have multiple sets of receivers and transmitterswhich operate generally as described herein to support hundreds orthousand of UE within a given cell. Each transmitter may be embodiedgenerally by a processor 1009 that executes instructions from memory1013 to perform the scrambling, mapping, and OFDM signal formation,using signal processing techniques as are generally known in the artalong with embodiments of the invention described herein.

Other Embodiments

While the invention has been described with reference to illustrativeembodiments, this description is not intended to be construed in alimiting sense. Various other embodiments of the invention will beapparent to persons skilled in the art upon reference to thisdescription. For example, a larger or smaller number of symbols thendescribed herein may be used in a slot.

While the invention has been described with reference to DLtransmission, it may be equally applied to UL transmission.

Embodiments of the invention may support single user dual-layerbeamforming using UE specific RS for both LTE-TDD and FDD.

The term “frame” and “subframe” are not restricted to the structure ofFIG. 2-4. Other configurations of frames and/or subframes may beembodied. In general, the term “frame” may refer to a set of one or moresubframes. A transmission instance likewise refers to a frame, subframe,or other agreed upon quantity of transmission resource.

Embodiments of this invention apply to various types of frequencydivision multiplex based transmission. Thus, the concept can easily beapplied to: OFDMA, OFDM, DFT-spread OFDM, DFT-spread OFDMA, SC-OFDM,SC-OFDMA, MC-CDMA, and all other FDM-based transmission strategies.

A NodeB is generally a fixed station and may also be called a basetransceiver system (BTS), an access point, or some other terminology. AUE, also commonly referred to as terminal or mobile station, may befixed or mobile and may be a wireless device, a cellular phone, apersonal digital assistant (PDA), a wireless modem card, and so on.

As described in general above, embodiment of the invention may performall tasks described herein such as channel monitoring and precodingselection, formation of transmission signals, etc, using logicimplemented by instructions executed on a processor. Another embodimentmay have particular hardwired circuitry or other special purpose logicoptimized for performing one or more to the tasks described herein.

An embodiment of the invention may include a system with a processorcoupled to a computer readable medium in which a software program isstored that contains instructions that when executed by the processorperform the functions of modules and circuits described herein. Thecomputer readable medium may be memory storage such as dynamic randomaccess memory (DRAM), static RAM (SRAM), read only memory (ROM),Programmable ROM (PROM), erasable PROM (EPROM) or other similar types ofmemory. The computer readable media may also be in the form of magnetic,optical, semiconductor or other types of discs or other portable memorydevices that can be used to distribute the software for downloading to asystem for execution by a processor. The computer readable media mayalso be in the form of magnetic, optical, semiconductor or other typesof disc unit coupled to a system that can store the software fordownloading or for direct execution by a processor.

As used herein, the terms “applied,” “coupled,” “connected,” and“connection” mean electrically connected, including where additionalelements may be in the electrical connection path. “Associated” means acontrolling relationship, such as a memory resource that is controlledby an associated port.

It is therefore contemplated that the appended claims will cover anysuch modifications of the embodiments as fall within the true scope andspirit of the invention.

1. A method for transmitting in a downlink channel of a cellularnetwork, the method comprising: selecting by a base station (eNB) totransmit either one or two transmission layers; when one transmissionlayer is selected: precoding a first transmission layer with a firstprecoder; selecting a first demodulation reference signal (DMRS)sequence or a second DMRS sequence and precoding the selected DMRSsequence using the first precoder; transmitting the first transmissionlayer with the selected precoded DMRS from the eNB to a user equipment(UE); and transmitting an indicator to the UE to indicate which DMRSsequence is selected and transmitted.
 2. The method of claim 1, whereinwhen two transmission layers are selected, comprising: precoding a firsttransmission layer with a first precoder, and precoding a first DMRSsequence using the first precoder; precoding a second transmission layerwith a second precoder, and precoding a second DMRS sequence using thesecond precoder; transmitting the first transmission layer and the firstDMRS sequence to the UE; and transmitting the second transmission layerand the second DMRS sequence to the UE.
 3. The method of claim 1,wherein the indicator is a one-bit indicator comprised in a downlinkgrant.
 4. Method for receiving in a downlink channel of a cellularnetwork, the method comprising: receiving at a user equipment (UE) arank indicator from a base station (eNB) indicating one transmissionlayer or two transmission layers; when one transmission layer isindicated: receiving an indicator from the eNB to indicate whichdemodulation reference signal (DMRS) sequence was selected by the eNBfrom a first DMRS sequence or a second DMRS sequence; receiving a firsttransmission layer from the eNB with the selected DMRS; and demodulatingthe selected DMRS and the first transmission layer, wherein the firsttransmission layer and the selected DMRS have a same precoding.
 5. Themethod of claim 4, wherein the indicator is a one-bit indicatorcomprised in a downlink grant.
 6. The method of claim 4, wherein whentwo transmission layers are indicated, comprising: receiving at the UEfrom the eNB a first transmission layer and a first DMRS having a firstsequence; receiving at the UE from the eNB a second transmission layerand a second DMRS having a second sequence; demodulating the first DMRSand the first transmission layer, wherein the first transmission layerand the selected DMRS have the same precoding; and demodulating thesecond DMRS and the second transmission layer, wherein the secondtransmission layer and the selected DMRS have the same precoding.
 7. Abase station for a cellular network, the device comprising: atransceiver configured to couple to a plurality of antenna, wherein thetransceiver is configured to transmit to and receiver from userequipment via the plurality of antenna in the cellular network; aprocessor coupled to a memory configured to execute a control program inthe memory for controlling the transceiver; wherein the transceiver isconfigured to select to transmit either one or two transmission layers;when one transmission layer is selected, the transceiver is configuredto: precode a first transmission layer with a first precoder; select afirst demodulation reference signal (DMRS) sequence or a second DMRSsequence and precode the selected DMRS sequence using the firstprecoder; transmit the first transmission layer with the selectedprecoded DMRS from the base station to a user equipment (UE); andtransmit an indicator to the UE to indicate which DMRS sequence isselected and transmitted.
 8. The base station of claim 7, wherein whentwo transmission layers are selected the transceiver is configured to:precode a first transmission layer with a first precoder, and precode afirst DMRS sequence using the first precoder; precode a secondtransmission layer with a second precoder, and precode a second DMRSsequence using the second precoder; transmit the first transmissionlayer and the first DMRS sequence to the UE; and transmit the secondtransmission layer and the second DMRS sequence to the UE.